Overhaul filesystem store to no longer use renameat2

This PR introduces a massive overhaul to remove the dependency on
renameat2 as well as fix bugs in the process. The bugs found are:
* unref() was not being called when insert() replaced an item
* Various extremely unlikely corner cases around files being
  created, replaced, deleted
* Edge case where the contents of a FileEntry could use the wrong
  data on data replacement (although very unlikely).

The paradigm of "FileEntry" entries and it's corresponding file
are now full immutable, meaning if the file gets replaced it
will generate a new FileEntry and shuffle the files around so both
may be read uninterrupted at any point.

fixes: 106

cas/store/BUILD

@@ -216,6 +216,7 @@ rust_library(
     srcs = ["filesystem_store.rs"],
     deps = [
         "//config",
+        "//third_party:async_lock",
         "//third_party:bytes",
         "//third_party:filetime",
         "//third_party:futures",
@@ -395,6 +396,8 @@ rust_test(
     deps = [
         "//config",
         "//third_party:filetime",
+        "//third_party:futures",
+        "//third_party:lazy_static",
         "//third_party:pretty_assertions",
         "//third_party:rand",
         "//third_party:tokio",